Porous webs comprising meltblown fibers are in widespread use in applications such as filtration of particulates and removal of oil from water, and as acoustic or thermal insulation. Such webs have been made from thermoplastic resins using melt-blowing techniques of the type described in Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled “Manufacture of Super Fine Organic Fibers” by Van A. Wente et al.
In the formation of such fiber webs, it has sometimes been found advantageous to use different polymers that have different characteristics. For example, U.S. Pat. No. 3,981,650 to Page describes a melt blowing die which is capable of simultaneously producing plastic filaments from two different polymers.
Meltblown fiber webs have also been made with different diameter fibers. For example, U.S. Pat. No. 5,783,011 to Barboza et al. discloses a filtration medium that is formed of a mass of nonwoven melt blown support and filtration fibers which are integrally co-located with one another. The support fibers have, on average, relatively larger diameters as compared to the filtration fibers.
In addition, U.S. Pat. No. 6,315,806 to Torobin et al. describes a composite filtration medium web of fibers containing a controlled dispersion of a mixture of sub-micron and greater than sub-micron diameter polymeric fibers.
U.S. Pat. No. 6,319,865 to Mikami describes a nozzle piece that gives melt-blown nonwoven fabric in one step, composed of fine fibers having a diameter in a range of from 1 to 10 μm.
So-called staple fibers have also been added to meltblown nonwoven webs. For example U.S. Pat. No. 6,827,764 to Springett et al. describes a filter element that comprises a porous molded web that contains thermally bonded staple fibers and non-thermally bonded electrically charged meltblown fibers.